When using the ResponseBuilder.lastModified method, the Last-Modified header is formated like this:
Thu Mar 26 09:28:26 CET 2009

This is not a correctly formatted HTTP date, thus is not parsable when it is returned by the browser in the If-Modified-Since header.
It seems like the date is not formatted by the org.jboss.resteasy.util.DateUtil but is simply produced by calling Date.toString().

A workaround to this problem is to use the ResponseBuilder.header method and format the date:
builder.header("Last-Modified", DateUtil.formatDate(new Date()));